Self-organization of ice-wedge systems during their formation and degradation

Contemporary understanding of the formation of ice wedges and polygonal systems are based on the theoretical thermo-mechanical models of Dostovalov (1957) and Lachenbruch (1962), which were later refined by Romanovskii (1977) and Ratz and Chernyshev (1970). These models of frost cracking and subsequent ice formation considered two scenarios. The first one considered an infinite semi-space, the other a quarter of semi-space with one free edge. It both cases, the area of consideration is suddenly exposed to a cold climate that leads to frost cracking. The initial models predicted a hexagonal shape of polygons under uniform soil conditions and deviations from hexagonal shape were explained by inhomogeneous soil properties. The models also predict formation of first cracks parallel to the shoreline. Our field observations are not consistent with such a prediction and show that the first generation of ice wedges commonly forms perpendicular to the shorelines due to self propagation of ice wedges formed at earlier floodplain stages in the surrounding areas. Similar processes occur in recently drained-lake basins and under shallow water. Self organization of ice-wedge systems also takes place during degradation of ice wedges as the degradation spreads laterally along the ice-wedge troughs during the following years. This lateral progression is a self-reinforcing process of lateral heat and water transfer rather than downward thawing in response to climate warming. Even when ice wedges are well protected from atmospheric warming, the lateral degradation of ice wedge can continue. These processes indicate that initial patterns of ice-wedge systems strongly contribute to the self organization during formation of new ice wedges and the sequential patterns of degradation are strongly controlled by the ice-wedge system itself.